Appparatus and method for controlling a centrifugal compressor

ABSTRACT

In an electric motor driven centrifugal compressor, apparatus and method for controlling the compressor to prevent &#39;&#39;&#39;&#39;surge&#39;&#39;&#39;&#39; and operation at a motor current above the maximum rated motor current and for automatically restarting the compressor after a short-term power failure. The current to the compressor motor is sensed as an indicator of impending &#39;&#39;&#39;&#39;surge&#39;&#39;&#39;&#39;. When the current falls below a pre-selected minimum value, a blow-off valve in the compressor outlet line is gradually opened to allow excess volume to escape to the atmosphere, thus avoiding &#39;&#39;&#39;&#39;surge&#39;&#39;&#39;&#39;. The current to the motor is also sensed as an indication of compressor operation above the maximum rated motor capacity. When the motor current rises above a pre-selected level, an inlet valve to the compressor is closed. This restricts the volume available for compression and prevents motor over-loaded operation. In the event of a short-term power failure or power dip, contact pairs controlled by time delay relays remain closed for a short period of time. These pairs provide parallel bypass circuits around the normal motor starting circuits and allow an automatic restart if the time period has not been exceeded. The compressor is powered by the flywheel effect of its main drive gear during these shortterm power failures. If the power failure is of a long enough duration, the bypass contact pairs open, and the normal restarting procedures must be followed.

United States Patent [191 Bauer, Jr.

[ Dec. 11, 1973 APPPARATUS AND METHOD FOR CONTROLLING A CENTRIFUGAL Primary Examiner-B. Dobeck Att0rneySteve M. McLary et al.

[57] ABSTRACT In an electric motor driven centrifugal compressor, apparatus and method for controlling the compressor to prevent surge and operation at a motor current above the maximum rated motor current and for automatically restarting the compressor after a short-term power failure. The current to the compressor motor is sensed as an indicator of impending surge. When the current falls below a pre-selected minimum value, a blow-off valve in the compressor outlet line is gradually opened to allow excess volume to escape to the atmosphere, thus avoiding surge. The current to the motor is also sensed as an indication of compressor operation above the maximum rated motor capacity. When the motor current rises above a pre-selected level, an inlet valve to the compressor is closed. This restricts the volume available for compression and prevents motor over-loaded operation. In the event of a short-term power failure or power dip, contact pairs controlled by time delay relays remain closed for a short period of time. These pairs provide parallel bypass circuits around the normal motor starting circuits and allow an automatic restart if the time period has not been exceeded. The compressor is powered by the flywheel effect of its main drive gear during these short-term power failures. If the power failure is of a long enough duration, the bypass contact pairs open, and the normal restarting Tprocedures must be followed.

17 Claims, 3 Drawing Figures PATENTEI] DEC 1 1 I975 SHEET 2 [IF 3 N 05 On mm w w: of

mam :amoFE n wmwzamoibqT 2 av Ev APPPARATUS AND METHOD FOR CONTROLLING A CENTRIFUGAL COMPRESSOR BACKGROUND OF THE INVENTION This invention generally relates to centrifugal compressors. More particularly, this invention relates to control systems for such compressors. Specifically, this invention relates to an integrated control system for a centrifugal compressor which will prevent surge, will prevent operation above the motor rated capacity, and will automatically restart the compressor after a short time duration power failure.

Centrifugal compressors have long been recognized as extremely useful devices and are in common use in industry. However, there are two very critical operational limits of centrifugal compressors which must be controlled. At one limit surge must be prevented. In the simplest sense, surge" is a reverse-forward cycling of the gas being compressed which occurs when the back pressure of the process becomes greater than the compressor pressure. To avoid this, a blow-off valve is provided in the compressor outlet to vent the compressor when the flow falls below a preset minimum. Conventionally, a differential pressure (or AP) cell in the compressor outlet line measured the pressure drop across the AP cell, converted this pressure drop to a flow rate, and opened and closed the blow-off valve as the pressure (or flow, as it was actually used) reached a set point indicating impending surge. This system was not altogether satisfactory since the AP cells were very sensitive to dirt, were not automatically compensated for the inlet gas temperature, and required careful selection and calibration for every compressor. In addition, operation of two compressors in parallel to feed a common process outlet line was extremely difficult. If the two compressors outlet pressures were not exactly the same, the AP cell of one compressor would be forced to read backward, falsely indicating a surge condition. This would cause the blow-off valve of that compressor to open unnecessarily, thus wasting compressed gas. In addition, the output of the other compressor would be partially vented through the open blow-off valve, again causing inefficiencies. The other critical limit occurs at the opposite extreme from surge". If the system restriction is very low, the compressor motor can draw sufficient current to allow operation above the rated current capacity of the motor. This will cause overheating of the motor and eventual shutdown due to overheating. In the prior art, an inlet valve was operated to limit the available inlet volume to the compressor. A pressure controller sensed the process pressure and controlled the inlet valve as a function of process pressure. The motor current was also sensed and fed, as a pressure signal, to a pneumatic flip-flop. The pressure controller signal was also fed to the flip-flop, whose output actually controlled the inlet valve. The flip-flop would always pass the lower of the two input signals into the inlet valve. When motor overcapacity operation became imminent, the signal from the motor would be lower, and the inlet valve would close. This would unload the motor, and the signal from the pressure controller would be passed. If can easily be seen that if the process condition which led to the motor overcapacity operation in the first place were not corrected, the flip-flop would then immediately pass the motor signal again, closing the inlet valve. This clearly led to an undesirable oscillatory type of control.

My invention will control both surge" and overrunning, as well as allowing parallel compressor operation, without any of the prior art. Finally, in the prior art, any power failure would force a compressor shutdown. This could be very disruptive to a sensitive process. I have found that my system will ride short duration power failures and allow automatic restart without shutdown when power is restored.

SUMMARY OF THE INVENTION This invention is an apparatus and method for controlling an electric motor driven centrifugal compressor to prevent operation at a volume above the maximum rate flow, to prevent the occurence of surge and to automatically restart the compressor after a short time duration power failure. One aspect of the invention is to limit the flow to no more than the maximum rated capacity of the compressor; to do this, an inlet valve is controlled partially as a function of motor current. The process pressure in the compressor outlet is sensed and a signal proportional to this pressure is generated. This proportional signal is compared with a preset value of process pressure and a control signal proportional to the deviation of the proportional signal from the preset value is generated. In addition, the motor current is compared to a preset maximum current value. So long as the current remains below the maximum, the control signal is passed to the inlet valve without modification. If the current rises above the maximum value, the control signal is modulated as a function of the amount above the preset maximum, thus controlling the inlet valve as a function of the control signal whether modified or not. Another aspect of the invention is to prevent surge by operating a blowoff valve in the compressor outlet piping as a function of motor current. The motor current is sensed and a signal proportional to the motor current is generated. This proportional signal is compared with a preselected minimum value for the proportional signal. If the proportional signal falls below the pre-selected minimum value, a modulated control signal is generated which opens the blow-off valve as a function of the modulated control signal. Yet another aspect of the invention'is to automatically restart the compressor after a short time duration power failure by providing time dependent bypass circuits around the normal means for initiating the motor start. If the power failure is less than a pre-selected time, the sequential operation will be automatically re-initiated without intervention of a human operator to thereby restart the motor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of the electrical control circuit of the present invention;

FIG. 2 is a schematic circuit diagram of the flow limiting and anti-surge controls of the present invention; and

FIG. 3 is a schematic circuit diagram of the motor starter circuit of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a ladder circuit diagram 10 for the electrical control system of the present invention. The ladder circuit 10 receives its power from a transformer 12, as is conventional practice. The configuration of the components in FIG. 1 is that assumed when the compressor is not running and power has been off for a long time. In addition, the notation used throughout in FIG. 1 will be that conventionally used in describing ladder diagrams. That is, the coil of a relay will be given a specific letter and number. Then, the contact pairs associated with that particular relay will be given the same letter and number as the coil and in addition will be given a unique identifying numeral. To start the compressor operating, the procedure is as follows: a momentary contact reset switch 14 is pressed by the operator. This, in turn, allows current to flow from side to side in the ladder diagram through a relay coil 3CR. It will be noted that a number of safety interlock elements 16, 18, 20, 22 llCRl and 2TR1 are located in series intermediate the reset switch 14 and the coil 3CR. Element 16, in the form of a switch, is used to bring the compressor to a complete halt when it is desired to shut the compressor off. In the startup procedure, the switch element 16 is closed. Element 18, in the form of a pressure actuated switch is connected to the compressor lubrication system and senses when the pressure of the lubricating oil is too low for proper functioning and opens to stop the compressor. This switch element 18 would normally be closed in the startup procedure indicating that the oil pressure is sufficient to allow operation to begin. Element 20, in the form of a temperature actuated switch, is located in the lubricating oil pressure line and senses the temperature of the lubricating oil. If the temperature of the lubricating oil becomes too high for proper operation, the switch element 20 opens, thus disrupting the circuit and causing the compressor to shut down. Again, in the normal startup procedure, the switch element 20 would be closed, indicating that the oil temperature was proper. Element 22, a second temperature actuated switch, is connected to the outlet of the compressor and senses the outlet air temperature. The switch element 22 is set such that when the temperature of the outlet air reaches a predetermined maximum value, the switch element 22 opens, again disrupting the circuit. In the startup procedure, the switch element 22 would be closed, again completing the circuit. A relay coil lCR (see FIG. 3), connected across the power input line to a motor starter 24 for a compressor motor 25, controls the element lCRl, in the form of a contact pair, in series with the elements 18, 20 and 22. If there is power into the motor starter 24, relay coil lCR is energized and the contact pair ICRl will be closed, again allowing current to flow to the relay coil 3CR. Finally, element 2TR1, in the form of a contact pair, is controlled by a coil 2TR of a time delay type relay. The function of the contact pair element 2TR1 will be described later in conjunction with the discussion of the coil 2TR. The functioning of the coil 3CR closes the contact pair 3CRI and locks in the safety reset circuit. Thus, if all the elements indicate that the compressor is in the proper mode for starting, the compressor start button may be pushed and the compressor will start. The coil 3CR additionally energizes the contact pair 3CR2 at the lead into the compressor start circuit. With the contact pair 3CR2 closed, the operator will now press a compressor start button 28 to continue the startup procedure. The current flowing through the closed contact pair 3CR2 and the start switch 28 must flow through a limit switch 30 which is closed when an inlet valve 46 is closed. The inlet valve 46 must be closed before the compressor can be started. Next, the current flows through a limit switch 31, which is closed when a blow-off valve 56 is fully open. The blow-off valve 56 must be in the fully open position before the compressor can be started. Finally, the signal must pass through a minimum oil temperature switch 32. The minimum oil temperature switch 32 is set such that the circuit will be open if the lubricating oil temperature has not reached a pre-selected minimum value. Finally, the flow of electrical current energizes a coil 4CR of a relay. Energizing the coil 4CR, in turn, closes a contact pair 4CR1 and thus bypasses the compressor start switch 28. At this point, the compressor start switch 28 may be released by the operator and the startup procedure will continue. In addition, it will be noted that bypassing the compressor start switch 28 additionally bypasses the limit switches 30 and 31 and the low oil temperature switch 32. This function is permissible, since these three functions are monitored only for startup procedure and have no effect on the running operation of the compressor. With the contact pair 4CR1I in the closed position, current may now flow through the contact pairs 3CR2 and 4CR] and thus keep the coil 4CR energized. The coil 4CR, in addition, operates a contact pair 4CR2 (see FIG. 3) in the motor starter 24. Closing the contact pair 4CR2 allows current to reach the motor 25, as will be explained later, thereby start ing the compressor. Energizing the coil 4CR additionally closes a contact pair 4CR3, admitting current to a coil 5TR of a time delay relay. The time delay re'lay with which the coil STR is associated is of the type which delays the closing of a contact pair STRl of the relay for a period of time after the coil STR has been energized. The purpose of the time delay is to allow the motor 25 to reach full speed before the control circuit is placed in operation. This allows the drive motor 25 to be started in an unloaded condition and be at operational speed before the inlet valve 46 and the blow-off valve 56 come under control. I have found that a delay of approximately twenty seconds is sufficient to allow this condition to be established. Twenty seconds after the coil STR has been energized, the contact pair STRl closes, thus energizing a coil SCR of a conventional relay. Energizing the coil 5CR closes a contact pair SCRl, locking the coil 5CR into the circuit. In addition, a contact pair 5CR2 is also closed by the coil SCR, energizing a portion of the control circuit of the compressor. In particular, a coil 48V is energized to open a solenoid valve 38 (see FIG. 2). In addition, a coil 6TR of a time delay relay is also energized at this time. The time delay relay with which the coil 6TR is associated is of the type which delays the opening of its contact pairs a preset length of time whenthe power is turned off to the coil 6TR. It will be noted that a contact pair 6TR1, controlled by the coil 6TR, is connected in parallel with the contact pair 4CR1. The relay, controlled by the coil 6TR, has been found to be most appropriately set to provide a delay of approximately five seconds before the contact pair 6TR1 will open after the coil 6TR is de-energized. In addition, a coil 9TR associated with another time delay relay is also energized by the closing of the contact pair 5CR2. This time delay relay, controlled by the coil 9TR, is provided to ensure that the compressor has been running a preset length of time, in this case approximately one minute, before the power interruption override function, to be described later, can be utilized. I have found that once the compressor has been running approximately one minute, itis assumed that the power source is stable and that the compressor is operating satisfactorily. Therefore, after a period of approximately one minute, the coil 9TR allows a contact pair 9TR1 to be closed. Closing the contact pair 9TRll energizes a coil 7TR of yet another time delay relay. The time delay relay controlled by the coil 7TR is again of the type which delays the opening of the contact pairs a preset length of time after the coil 7TR has been de-energized. It will be noted that a contact pair 7TR1 is connected in series with the contact pair 6TR1, which, in turn, are in parallel around the contact pair 4CR1. I have found that the delay to be provided by the coil 7TR may most appropriately be set at approximately ten seconds. A normally closed contact pair 4CR4 controlled by the coil 4CR has been caused to open at the time of energization of the coil 4CR. This causes a loss of power to a coil 5SV, which permits a normally closed solenoid valve 40 to close. When power is initially supplied to the ladder circuit 10, a coil lTR of a time delay type relay is energized. The characteristics of the time delay relay controlled by the coil lTR are such that a contact pair lTRl will open a short period of time after the coil lTR has been energized. Thus, after an initial current flow to the coil lTR, the contact pair lTRl will be open. Initially, however, the contact pair lTRl will allow current to flow to the coil 2CR. The current to the coil 2CR, in turn, will close the contact pair 2CR1 connected in parallel around the contact pair 3CR1. In addition, a coil 2TR controlling another time delay relay is also energized during the time the coil 2CR is energized. The characteristics of the coil 2TR are such that the contact pair element 2TRl will open a short period of time after the coil 2TR has been energized. Thus, if for any reason, the time delay function generated by the coil lTR should fail and allow the contact pair ITRI to remain closed, the coil 2TR will operate to open the contact pair element 2TR] and halt the operation of the compressor. The coil 2CR, when it is energized, will function to open a contact pair 2CR2 and a contact pair 2CR3. The contact pairs 2CR2 and 2CR3 are normally closed and open only while power is initially provided to the ladder circuit 10. The flow of current through the contact pair 2CR2 controls a coil 2SV which causes a normally closed solenoid valve 42 (see FIG. 2) to open while power is on the circuit. The current flowing through the contact pair 2CR3 controls a coil 38V which is associated with a normally open" solenoid valve 44 (see FIG. 2). Therefore, so long as current is provided to the ladder circuit 10, the solenoid valve 44 will remain closed. Finally, a contact pair 6TR2 is connected in series with a contact pair 2CR1, and a contact pair 6TR3 is connected in parallel around the contact pair SCR]. In Table I, under the column labeled Normal Operation", the conditions of the contact pairs and elements in FIG. I; are tabulated for the normal power on operation.

TABLE I Contact Fairs and Normal Power Elements Operation Failure l4 Open Open JCR I Closed Open ZCRI Open Open 6TR2 Closed Closed 1'6 Closed Closed I8 Closed Closed Closed Closed 22 Closed Closed lCRl Closed Open 6 2TR] Closed Closed lTRl Open Closed 2CR2 Closed Closed 2CR3 Closed Closed 4CR4 Open Closed 3CR2 Closed Open 28 Open Open 4CR1 Closed Open 7TR] Closed Closed 6TR1 Closed Closed 30 Indeterminate Indeterminate 31 Indeterminate Indeterminate 32 Closed Closed 4CR3 Closed Open STRI Closed Open 5CR1 Closed Open 5CR2 Closed Open 9TR1 Closed Open 6TR3 Closed Closed Before considering the control system illustrated in FIG. 2, it is necessary to point out a few basic factors relating to the operation of centrifugal air compressors. For those desiring more information on the specific operation of centrifugal air compressors, reference may be had to Chapter 3 of Compressed Air and Gas Handbook, 3rd Edition, 1961, Compressed Air and Gas Institute. The operational characteristics and, consequently, the control requirements of a centrifugal type air compressor and a reciprocating type air compressor are considerably different. The reciprocating type air compressor is essentially a constant volume output, variable pressure output device. A centrifugal compressor, on the other hand, is a variable volume output, constant pressure output type device. In the field of operation herein discussed, the centrifugal compressor is operated at essentially a constant speed by a drive motor. Operating at this speed, the compressor will deliver a variable capacity of air from approximately fifty percent to one hundred percent of the rated capacity at substantially a constant discharge pressure. There are two limits or extremes within which the operational range of the centrifugal compressor must be controlled. Heretofore, the control of these two extremes of the range of operation has been somewhat erratic, and the control system of the present invention has succeeded in accurately controlling both limits of the operational range of the centrifugal compressor. At one extreme, the restriction to the output of the centrifugal compressor becomes so low that the compressor may take in amounts of air grossly in excess of the rated capacity. When this occurs, the compressor motor may overload itself and thereby be burned up. This function has normally been in control by the use of an inlet control valve, but the control of this valve has not been completely integrated with the control to the compressor output. At the other end of the compressor operating range, a phenomenon known as surge may occur. Quoting from the Compressed Air and Gas Handbook, at p. 322, the phenomenon of surge can be explained as follows:

At a particular speed, as the flow is reduced and the compressor back pressure (which is the result of all the restrictions) is increased or kept the same, a flow point is reached where the back pressure exceeds the pressure ratio developed by the compressor, and a breakdown in the flow results. This immediately causes a reversal in the flow direction and reduces the compressor back pressure. The moment this happens regular compression is resumed and the cycle is repeated. The cycling action has been properly labeled as surge.

lt may be appreciated that the occurrence of surge" can be very costly to a process and, if allowed to continue, will result in damage to the compressor itself. In order to prevent the occurrence of surge, a blow-off valve is generally provided in the outlet piping of the compressor. In prior systems, when surge occurs, the blow-off valve is opened to allow pressure to be vented to the atmosphere and increase the flow through the compressor. The operation of the blow-off valve has not been fully integrated with the total compressor circuit, thereby leading to inefficiencies in the operation of the compressor by allowing the blow-off valve to open, wasting air unnecessarily or leading to the shutdown of the compressor if surge occurs.

Turning now to the control circuit of FIG. 2, atmospheric air is furnished through the inlet valve 46 to an inlet pipe 47. The inlet valve 46 may be a Fisher Model 7810 with a 656 diaphram actuator manufactured by the Fisher Control Company of Marshalltown, Iowa 50158. The operation of the inlet valve 46 is such that it will be open an amount determined by the pressure of air signal furnished to it. The inlet valve 46 is set such that when it is fully closed, admission of atmospheric air through the inlet pipe 47 is not completely blocked. This allows the compressor to start while the inlet valve 46 is essentially in the closed position. Atmospheric air passing through the inlet valve 46 is then introduced to compressor 48 through a compressor inlet pipe 49. The compressor 48 may be any centrifugal type compressor currently available. For example, the compressor 48 may be a Clark lsopac 4, manufactured by Dresser Industries, Incorporated, Clark Turbo Compressor Division, Olean, New York 14760. For the purposes of this explanation, consider that the compressor 48 will give an output of compressed air at approximately fifty pounds per square inch gauge pressure. This compressed air is furnished to an outlet pipeline 50. A check valve 52 is provided in the outlet pipeline 50 to prevent gross reverse flows of air through the pipeline 50 to the compressor 48. Upstream of the check valve 52, in the pipeline 50, is a blow-off branch pipeline 54 connected to the flow-off valve 56. The blow-off valve 56 may be a Fisher Model EK with a 657 actuator manufactured by Fisher Controls, Marshalltown, lowa 50l58. The characteristics of the blow-off valve 56 are such that its degree of opening is governed by a signal pressure furnished to the operating mechanism of the blow-off valve 56. Under normal operating conditions, the blow-off valve 56 is completely closed. The blow-off valve 56 is open only under conditions where surge in the system becomes imminent. The output of the blow-off valve 56 is vented to the atmosphere through an atmospheric vent line 58. A source 60 of control air under pressure furnishes control air to a pipeline 61 to a control air manifold 62. A pressure sampling line 64 is connected to the outlet pipeline 50 to sense the process pressure. The pressure sampling line 64 is connected to a pressure transmitter 66, which may be a Foxboro Model M45, manufactured by the Foxboro Company, Foxboro, Massachusetts 02035. An additional input to the pressure transmitter 66 is through a control airline 68 from the control air manifold 62. The pressure transmitter 66 functions so as to present an output, through an output pipeline 70, of a control air pressure signal proportional to the pressure transmitted by the pressure sampling line 64. Advantageously, the control air pressure will be in a range of twenty pounds per square inch gauge. The output of the pressure transmitter 66 will advantageously be of the range of from three to fifteen pounds per square inch gauge, again proportioned relative to the actual process pressure. The output line 70 from the pressure transmitter 66'is connected to a pressure controller 72. The pressure controller 72 may be a Foxboro Model M/52A-7M4-B manufactured by the Foxboro Company, Foxboro, Massachusetts 02035. An additional input to the pressure controller 72 is a control airline 74 connected to the control air manifold 62. The pressure controller 72 is preset to the desired process pressure. As the process pressure varies, the pressure controller 72 will generate a corrective signal and transmit this signal through an output line 76. The process pressure, as was previously explained, is actually transmitted as a proportional signal by the pressure transmitter 66 to the pressure controller 72. The pressure controller 72 compares this proportional signal with the preset value and produces an output of control air in the output line 76 proportional to the deviation from the actual set point pressure. The output signal, from the pressure controller 72, in the output line 76, is a modulated pressure signal. This modulated pressure signal transmitted by the output line 76 is connected to a current-to-pressure converter 78. The current-to-pressure converter 78 may be a Model 775 with reverse acting transducer, manufactured by Moore Products Company of Spring Hill, Pennsylvania 19477. The function of the current-to-pressure converter 78 is to prevent the compressor motor 25 from operating at a level above its maximum rated capacity. A coil 80 in one phase of the input wiring to the motor 25 senses the current drawn by the motor 25. It is well known that a measure of the work being done by any motor may be the current being drawn by the motor. Thus, as the compressor motor 25 begins to operate at higher and higher current values, the motor may overheat and eventually trip the motor overload safetys, thus shutting down the compressor 48. To ensure that this can never occur, the current in the wiring to the motor 25 is transmitted to the current-to-pressure converter 78. So long as the current sensed by the coil 80 and transmitted to the current-to-pressure converter 78 is below that current which would cause operation of the compressor 48 at an inlet volume above the maximum drive motor 25 rating, the control signal from the pressure controller 72 passes through the current-to-pressure converter 78 without deviation. However, as the current reaches the maximum level, the current-topressure converter 78 functions to modulate the pressure output in the output line 76 to a level which will cause the inlet valve 46 to assume a position such that the maximum inlet volume may not be exceeded. The output from the current-to-pressure converter 78 is transmitted to an output line 82 through a solenoid valve 38 and thence via an output line 83 to a volume booster 84. The volume booster 84 may be a Foxboro Model BIOZRN, manufactured by the Foxboro Company, Foxboro, Massachusetts 02035. The volume booster 84 receives an input from the control air manifold 62 through a pipeline 86. The function of the volume booster 84 is to increase the volume of the control signal coming in through the pipeline 83 to a level which will operate the inlet valve 46 quickly. That is, the inlet valve 46 is positioned according to the pressure signal received by the inlet valve 46 in an input line 88 from the volume booster 84. If the volume booster 84 were not utilized, the output pressure in the line 76 could be used to operate the inlet valve 46, but the operational speed of the inlet valve 46 would be relatively slow. Thus, the volume booster 84 allows more rapid operation of the inlet valve 46. It will be noted that there is a bypass around the current-to-pressure converter 78. This bypass line is made up of a pipeline 89 connected directly to the output line 76 from the pressure controller 72. The pipeline 89 is connected to a solenoid valve 44. The solenoid valve 44, in turn, is connected to the output line 83 with a pipeline 90. Under normal operational conditions, the solenoid valve 44 is closed, as previously discussed, and the bypass line is not in use. The bypass line comes into play only during power failures, a function which will be described later in detail. In addition, provision is made to allow any residual pressure in the output line 83 to be blown off to the atmosphere during periods of no operation. This function is carried out by a blow-off line 92 connected to the solenoid valve 40, which then has an atmospheric exhaust line 93 connected to its output.

A second and independent control circuit shown in FIG. 2 controls the operation of the blow-off valve 56 to prevent surge from occurring withinthe system. As was previously explained with respect to the maximum volume limiting system, it is likewise possible to sense the imminence ofsurge" in the system by monitoring the current being drawn by the motor 25. For

, any given compressor, surge will occur at a unique value of current drawn by the compressor motor 25. This point may be determined for any system by simple experimentation before the system is placed on stream. A current-to-pressure converter 96 receives control air through a pipeline 97 from the control air manifold 62. The current-to-pressure converter 96 may be a Model 775 with forward acting transducer manufactured by the Moore Products Company of Spring House, Pennsylvania 19477. A coil 98 senses the current in one .phase of the input electrical wiring-to the compressor motor 25 and transmits the current signal to the current-to-pressure converter 96. The current-to-pressure converter 96 continuously transmits a pressure signal proportional to the current flow to the compressor motor 25. The pressure signal is transmitted through a pipeline 99 to the solenoid valve 42 and thence through a pipeline 100 to a flow controller 102. The flow controller 102 may be a Foxboro Model M/52A-7M4-B, manufactured by the Foxboro Company, Foxboro, Massachusetts 02035. The flow controller 102 receives a control air input from a pipeline 104 connected to the control air manifold 62. The function of the flow controller 102 is to take the proportional signal pressure transmitted by the current-to-pressure converter 96 and convert this signal into a pressure signal recognizable by the blow-off valve 56. That is, the signal pressure transmitted by the current-to-pressureconverter 96 would not necessarily be of the proper range to operate the blow-off valve to avoid surge. Therefore, the flow controller interrupts the signal transmitted by the current-to-pressure converter 96 and transmits a signal through an output pipeline 106 which will operate the blow-off valve 56. The flow controller 102 is preset to give a maximum output pressure signal, so long as the proportional signal pressure signal from the current-to-pressure converter 96 remains above a value indicating normal operation. This allows the blow-off valve 56 to be closed, since, as previously noted, the blow-off valve 56 is normally open and requires a pressure signal to keep it closed. However, as the signal from the current-to-pressure converter 96 begins to fall below the pre-selected level, the pressure output from the flow controller 102 to the blow-off valve 56 will begin to drop, and the blow-off valve 56 will start to open. The output line 106 is connected to a volume booster 108. The volume booster may be a Foxboro Model BIOZRN, manufactured by the Foxboro Company, Foxboro, Massachusetts 02035. The volume booster 108 receives a source of control air through a pipeline 110 connected to the control air manifold 62. As previously explained, the only function of the volume booster 108 is to increase the volume of the signal transmitted by the flow controller 102 through the pipeline 106. This is to allow operation of the blow-off valve in a more rapid manner than would be possible if the pressure signal in the pipeline 106 were used. The output of the volume booster is transmitted to the blow-off valve 56 through an output pipeline 112. As the current sensed by the coil 98 varies in the range in which surge" is impending, the degree of opening of the blow-off valve 56 is controlled by the modulated signal received from the pipeline 112 to open and close the blow-off valve 56 in such a manner that surge would be prevented. So long as the operation of the entire system is in the pre-selected range, the blow-off valve 56 will be modulated open and closed. As surge becomes more imminent, as indicated by the current sensed by the coil 98, the blow-off valve will become further and further open until, just prior'to the surge point the blow-off valve 56 will be fully open. With the blow-off valve 56 in the fully open position, it is impossible for surge to occur, since a vent is provided for the minimum capacity volume generated by the compressor 48. As the conditions which were tending to create surge in the system correct themselves, the current sensed by the coil 98 will rise above the value which indicated the imminence of surge, the blow-off valve 56 will become fully closed again, and the system will be operating in a normal mode. Provision has also been made to allow the system to continue operating, even in the event of a failure of the source 60 of control air. A pipeline 113 is connected to the pressure sampling line 64. The pressure in the pipeline 113 will be whatever the system pressure is, and generally will be too high for use as control air. Thus, the pipeline 113 is connected to a pressure regulator 114, which reduces the pressure to a value slightly less than that of the control air source 60. The pressure regulator 114 is connected to a check valve 116 by a pipeline 115. The check valve 116 is connected to the line 61, into the control air manifold 62, by a pipeline 117. So long as air is being furnished by the source 60 through the pipeline 61, the balance of pressures in the check valve 116 will prevent air from flowing through the pipeline 1 15 into the pipeline 61. However, in the event of a failure of the pressure from the source 60, the check valve 116 will open and allow air, reduced in pressure by the pressure regulator 114, to flow through the pipeline 117 into the pipeline 61 and thus continue to furnish air through the control air manifold 62. A check valve 59 interposed in the pipeline 61 between the control air manifold 62 and the control air source 60, prevents any backflow of air into the dead control air source 60 during such periods. The scheme just described for continuing operation in case of a failure of the source 60 of control air will also allow operation of the compressor 48 with no source of control air whatsoever. That is, under some circumstances, a source 60 of control air is not available, as in the case where a single compressor 48 is in use. Thus, the compressor 48 is initially started as previously described. The startup procedure requires the inlet valve 46 to be closed and the blow-off valve 56 to be open. In this configuration, the compressor 48 is in a stable operating mode. However, as soon as the. compressor 48 begins to generate a system pressure, the pipeline 113 will begin to receive pressure through the pressure sampling line 64. When this occurs, the pressure may be then further transmitted to the control air manifold 62. Once the control air manifold 62 has been pressurized, the other elements of the anti-surge and maximum flow limiting control circuits may be energized from the control air manifold 62, allowing the blow-off valve to be closed and the inlet valve to be opened under control. In this situation, it is again emphasized, there is no source 60 of control air whatsoever present. The only source of control air is that furnished by the compressor 48 itself as it begins to operate.

The solenoid valve 40 is of the normally closed type. Thus, when power is first applied to the ladder circuit 10, the valve 40 will be open (receiving power through the normally closed contact pair 4CR4) and vent any residual pressure in the line 83 to the atmosphere. As the startup proceeds, the coil 4CR will be energized, thus opening the contact pair 4CR4, closing the valve 40. During a power failure, the valve 40 will remain closed, because no power is available in the ladder circuit to energize the coil SSV. When the power is restored, the valve 40 will be open for a very short period of time until the coil 4CR can be re-energized.

It should be pointed out that the surge prevention system is essentially self-compensating for changes in the ambient temperature of the gas being compressed. In the prior art systems using AP cells, the surge point could shift toward or away from the set point as the temperature varied, since surge is, in part, a volume phenomenon and thus a function of ambient gas temperature. In the present invention, the current to the motor 25 will rise and fall as the gas temperature rises and falls, since the work required to compress a fixed volume will depend on the gas density. Since surge" in this invention is sensed as a function of motor current, the floating motor current compensates for the change in the surge" point due to temperature variations.

In addition to completely controlling surge and overloading the compressor motor 25, the control system of the present invention will also allow the compressor 48 to survive short duration power interruptions without any significant loss of process pressure of the necessity of shutting the compressor 48 completely down and restarting it. Heretofore, it has been believed impossible to allow centrifugal compressors to remain in an operating mode during short periods of power interruptions. It has been the conventional practice to provide safety interrupt circuits in the main power control panel to force the compressor to shut down whenever the power fails, or due to conventional control schemes. This necessitates going through the entire laborious startup procedure to bring the compressor back on line, as well as a considerable loss of production. I have found that by proper design of the circuit as herein disclosed, the compressor may safely ignore power dips or failures of up to five seconds in duration and remain in the running mode. The power input, to maintain the compressor operation during these power dips, is provided by the main input drive gear to the compressor 48 which acts as a flywheel during these short power interruptions. The circuit which allows the restarting of the compressor 48 once the power is rerestablished without an operator or necessity of completely going through the startup procedure is as follows: Reference should first be made to Table 1 under the column labeled Power Failure. This listing establishes the condition of each contact pair within the ladder diagram 10 shown in FIG. 1. Particular note should be paid to the contact pairs 6TR2, 7TR1, 6TR1 and 6TR3. It should be apparent from Table 1 that these terminals remain in the on condition, even through power has failed. This results from the characteristics of the coils 6TR and 7TR, which hold the contact pairs closed for a short period of time after the coils have been re-energized. To date, l have found that this period of time may most advantageously, be held to around five seconds in duration before the flywheel effect of the compressor 48 main drive gear begins to dissipate sufficiently to cause difficulties in restarting of the compressor 48. Assuming now the terminal configuration described in Table 1 under thePower Failurecolumn, it will be noted that the coils 2SV, 38V and 4SV controlling the solenoid valves 44, 38 and 42 have been affected by the power failure. ln particular, the solenoid valve 42 has been closed, the solenoid valve 38 has been closed, and the solenoid valve 44 has been opened. It should have been apparent that the control system for the compressor 48 which controls the inlet valve 46 and the blow-off valve 56 is largely pneumatic in nature and self-contained. Thus, an electrical power failure does not necessarily disrupt the ability of the inlet valves 46 and the blow-off valve 56 to operate. Closing the solenoid valve 42 looks whatever pressure signal was being introduced into the flow controller 102 through the inlet line 100 at the time of the power failure. This means that the pressure signal will remain constant to the blow-off valve 56 during the time of power failure and the blow-off valve 56 will remain in the position it had assumed just prior to the power failure until power is re-established. Likewise, when the solenoid valve 38 closed, the path from the inlet valve 46 to the pressure controller 72 was blocked. However, opening of the solenoid valve 44 provided a bypass through the pipelines 89 and 90 through the volume booster 48 and to the inlet valve 46. This allows the inlet valve 46 to be modulated during the periods of power failure through the pressure transmitter 66 and the pressure controller 72. The current-to-pressure converter 78 is effectively bypassed during this period of time, but since there is no electrical power, the coil 80 could not generate a signal to the current-topressure converter 78 and consequently it is of no influence whatsoever during power failure periods. It should be again kept in mind that the duration of the power failures which are herein discussed are generally of a short duration, five seconds or less. Thus, the time during which the control of the inlet valve 46 and the blowoff valve 56 is strictly under pneumatic control is of a relatively short duration. The provision for the control during this period of time, however, allows the compressor 48 to be restarted in a substantially stable operational mode and prevents the development of large oscillations as the control system again takes effect with the resumption of electrical power. Considering the ladder diagram of FIG. 1 with the power off and assuming now that power is re-established after a short duration power failure or dip, the first devices which can receive power are the coils llTR and ZCR. It will be noted that the control pairs 3CRl and and 2CR1 prevent power from entering the safety reset circuit, and the open contact pair 3CR2 prevents power from entering the motor starting circuit. The coil lTR opens the contact pair lTRl after a predetermined time. When closed, the contact pair lTRl energizesthe coil 2CR which, in turn, energizes the contact pair 2CR1. In addition at this time, the coil 2TR is likewise energized. It can be seen that closing of the contact pair ZCRl in conjunction with the contact pair 6TR2, which it will be recalled remains closed during the short duration power failures, effectively bypasses the reset switch l4 and allows current to flow through the elements l6, 18, and 22. In addition, the contact pair lCRl is now closed since power is re-established to the main power inputs to the motor starter 24. The contact pair ZTRl likewise remains closed provided lTRl functions correctly. Finally, the incoming current will energize the coil 3CR as is the case in a normal procedure. At this point, the contact pair 3CR1 will again be locked in and the motor starting circuit will be energized just as if the reset button 14 had been pushed by the operator. Approximately two seconds after the coil lTR has been energized, the contact pair llTRl will open, thus cutting the power to the coil 2CR and the coil 2TR. This will open the contact pair 2CR1 as a safety precaution. In addition, the contact pair 2TR1 will remain closed since the current to the coil 2TR was stopped before the contact pair 2TR1 could be dropped out. The coil ZTR is provided strictly as a safety measure in case the contact pair lTRl should stick and not open properly as it should, thus leaving the contact pairZCRl closed and allowing the possibility of a false restart. In addition, the inrushing current will flow through the contact pairs 2CR2 and 2CR3, thereby opening the solenoid valve 42 and closing the solenoid valve 44 after the coil 2CR has been deenergized. Energizing the coil 3CR will close the contact pair 3CR2. It may be seen that there is now a parallel bypass around the contact pair 4CR1 and the start button 28 through the contact pairs 7TR1 and 6TR1. Again, it should be kept in mind that the contact pairs 7TRll and 6TRl are controlled by relays which hold them closed a preset length of time after power has failed to the coils 6TR and 7TR. The current may now flow through and energize the coil 4CR. Energizing the coil 4CR again closes the contact pair 4CR1, and power is established to the motor by closing the contact pair 4CR2. Likewise, the contact pair 4CR3 is closed and power is re-established to the time delay relay circuit STR and to the coil SCR through the parallel circuit of the contact pair 6TR3. Energizing the coil SCR further closes the contact pair SCRI and the coil SCR is again locked into the circuit. The contact pair SCRZ is then closed, thus allowing power to the coils 48V, GUI and QTR. Energizing the coil 48V opens the solenoid valve 38 and again allows control of the inlet valve 46 through the control system previously described. Energizing the coil 6TR then resets the contact pairs 6TR1, 6TR2 and 6TR3 in preparation for any future power failures. Additionally, coil 9TR, having been re-energized, begins again the timing cycle before the contact pair 9TR1 will be closed. The time period of the coil 9TR is much longer than the lime period of the coil 7TR. Thus, the contact pair 7TR1 will open before the coil 9TR has completed the timing cycle. It will therefore be impossible to restart the system if another power failure occurs within a relatively short period of time after the first. It is assumed that such rapidly occurring power failures indicate serious problems with the electrical supply, and the compressor 48 should be shut down in any event. Assuming, however, that a power failure does not occur within a short period of time, in this case sixty seconds, the contact pair 9TR1 will close once again, thereby energizing the coil 7TR. Energizing the coil 7TR will once again close the contact pair 7TR1 and allow the parallel bypass of the contact pair 4CR1 to be once again completed and ready for operation in the event of a short duration power failure.

To avoid the introduction of excessively high electrical signals to the current-to-pressure converters 96 and 78 during the initial startup procedure or during the restarting of the motor 25 after a power failure, a series of shorting contact pairs are provided in parallel in the electrical wiring leading from the coil 98 to the currentto-pressure converter 96 and the wiring leading from the coil 80 to the current-to-pressure converter 78. Considering first the circuitry from the coil 98 to the current-to-pressure converter 96, a normally closed contact pair 5CR3 is provided in parallel with the coil 98. Until the coil SCR has been energized, the contact pair 5CR3 will remain closed and provide a short circuit for any current in the coil 98. Once the coil SCR has been energized during the startup procedure, the contact pair 5CR3 is open and thus eliminates the short circuit. A second contact pair 2CR4 is connected in parallel with the terminal 5CR3. The normally open contact pair 2CR4 "is closed only during periods in which current is flowing to energize the coil ZCR. Thus, during the startup procedure, the coil ZCR is energized for a short period of time, causing the contact pair 2CR4' to be closed and provide a short circuit. In addition, during the time of restart of the motor 25 after a power dip, the contact pair 2CR4 is again closed by the brief energization of the coil 2CR during the restart procedure. This is necessary during the restart procedure after a power failure because the motor 25 continues to rotate even during the power failure, and once power is established, the motor must re-establish its phase relationship with the input voltage. This period of re-establishment of phase relationship results in relatively large currents being generated in the coil 98. These currents must be withheld from the current-topressure converter 96 to avoid distorted readings and consequent erratic operation of the blow-off valve 56. A normally closed contact pair 5CR4 and a normally open contact pair ZCRS are provided in parallel in the electrical wiring from the coil 80 to the current-topressure converter 78. These two contact pairs 5CR4 and ZCRS function identically with the previously described contact pairs 5CR3 and 2CR4 to prevent the introduction of excessively high currents to the current-to-pressure converter 78. In this case, the excessively high currents might result in erratic operation of the inlet valve 46.

FIG. 3 illustrates in schematic form the general configuration of the motor starter 24. Incoming threephase power is furnished to the motor starter 24 by three inlet electrical lines 118, 119 and 120. A transformer 122 is connected to the incoming wiring 118 and 120. The transformer 122 furnishes power to an electrical ladder circuit ll24. As previously discussed, connected in parallel across the ladder circuit 124 is a coil lCR whose functioning has been discussed in conjunction with the contact pair i'cRi. This coil icR ensures that main power is available to the motor. A second parallel branch contains the contact pair 4CR2 in series with a coil MR. When the contact pair 4CR2 is closed, the coil MR may likewise be energized. Energizing the coil MR closes the contact pairs MR1, MR2 and MR3. This then allows power to flow to the motor 25 through the main output lines 126, 127 and 128.

This control system has found particular utility in glass container manufacturing plants. Glass container manufacturing machinery. is almost entirely air operated. Thus, either surge or the shutdown of the plant air compressors due to a power failure or power dip is extremely costly. For example, a power failure leading to compressor shutdown of three seconds in a typical plant generates a production loss of $1 1,000. One other plant typically experienced eighteen such shutdowns per year. Most of the power failures were of the very short duration type which this control system successfully ignores. With the present control system, shutdowns due to short duration power failures have been virtually eliminated. As an example, one plant suffered a total of approximately twenty-six short-term power interruptions in one day due to an ice storm. The present control system allowed the compressors to successfully ride" all of these power interruptions without a single compressor shutdown or surge.

It may be of value in a fuller understanding of the present invention to break down the components previously described into sub-components with functional designations to better understand their interrelationship. For example, the ladder diagram 10 is actually made up of a number of sub-circuits which can be readily identified and labeled by function. For example, the circuit which includes the reset switch 14, the contact pair 3CRl, the safety interlock elements 16, 18, 20, 22, lCRl and 2TR1, and the coil 3CR may be labeled or termed a safety interlock circuit. It is the function of this circuit to monitor the status of critical components of the compressor 48 and prevent starting or continued operation of the motor 25 in the event of an abnormal status of any one of the critical components thus monitored. The reset switch 14 and the circuit locking contact pair 3CR1 provide parallel paths for power to enter the safety interlock circuit thus described. With the circuit thus described, a power failure would result in an inability for power to re-enter the circuit, since the locking contact pair 3CRl would become open at the time of a power failure and the reset switch 14 requires manual operation for it to function. However, the contact pairs 2CR1 and 6TR2, connected in parallel around the reset switch 14 and the locking contact pair SCRI, provide a separate circuit which may be described or termed as a safety interlock restart circuit. This safety interlock restart circuit allows bypassing of the previously described parallel components so that power may be re-introduced into the circuit after a short-term power failure. The circuit which includes the contact pair 3ClR2, the start switch 28, the locking contact pair 4CR1, the limit switches 30 and 31, the minimum oil temperature switch 32 and the coil 4CR may be considered to be an enabling circuit. The purpose of the enabling circuit is to allow the motor starter 24 to receive a signal which will allow current to be delivered to the motor 25. Again, it should be realized that upon a power failure, the enabling circuit may not be automatically restarted due to the presence of the open contact pair 3CR2, the open start switch 28 and the open circuit locking contact pair 4CRl. The contact pairs 7TR1 and 6TRH' connected in parallel with the locking contact pair 4CR1 constitute an enabling restart circuit which allows the enabling circuit to be re-energized after a power failure. The sub-circuit in the ladder diagram 10, which includes the contact pairs 4CR3, STRl, SCRll, 6TR3, 5CR2 and 9TR1l, and the coils STR, SCR, 6TR, 7TR and 9TR may be considered to be a power dip timing circuit. That is, it is the function of this circuit to time the duration of a power dip and determine whether or not the compressor 48 may be restarted without the necessity of going through the normal startup procedure. This timing function, as previously described, is primarily carried out by the coils 6TR and 7TR, but the entire circuit described is desirable to make the power dip timing circuit function in its intended manner. As a final sub-circuit, the coil llTR, the coil 2CR, the coil 2TR and the contact pair lTRl make up a redundant, fail-safe restart circuit. The function of all of these components and their interrelation has been previously described, so it is believed unnecessary to completely redescribe these functions again. However, the name given to this circuit, it is believed, is sufficiently descriptive of its function to allow one to understand the interaction of this circuit with the other sub-circuits just defined. Of the sub-circuits defined, the safety interlock restart circuit, the enabling restart circuit, the redundant fail-safe restart circuit and the power dip timing circuit all function together to make upa circuit means which is interconnected with the safety interlock circuit and the enabling circuit for automatically reenergizing the safety interlock circuit and the enabling circuit when the duration of a failure of the main electrical supply is of a duration less than a pre-determined period of time. The re-establishment of these connections will of course allow power to be again supplied to the drive motor 25 and will consequently cause a restart of the compressor 48 without intervention of an operator.

In a similar manner, it may be helpful to consider the control system for preventing compressor surge" and the control system for Preventing overrunning of the compressor motor 25, in a functional sense. The elements making up this control scheme have been described as pneumatically operated components. However, in a broader sense, these elements may be electriwith a preselected process pressure value and generating a control signal which represents the deviation of the proportional signal from the pre-selected process value. Finally, the current-to-pressure converter 78 may be considered to be a means which is responsive to the electrical current delivered to the motor 25, which will pass the control signal without deviation so long as the current value remains below a pre-selected value, and will modulate the control signal as a function of the motor current above the pre-selected value to adjust the inlet valve 46 in response to the modulated control signal to avoid exceeding the maximum permissible flow rate of the compressor 48. Similarly, in the surge" avoidance control scheme, the current-topressure converter 96 may be considered to be a means responsive to the electrical current delivered to the electric motor 25 for generating a signal proportional to the motor current. The flow controller 102 may then be considered a control means for comparing the generated proportional signal with a pre-selected minimum value of the proportional signal and generating a modulated control signal to open the blow-off valve 56 as the proportional signal falls below the pre-selected value and close the blow-off valve 56 as the proportional signal rises above the pre-selected value.

What I claim is:

1. Apparatus for limiting the current to an electric motor driving a centrifugal compressor to a preselected maximum current, comprising, in combination: An adjustable, flow-limiting inlet valve connected to the inlet side of said centrifugal compressor; signal generating means connected to the outlet side of said centrifugal compressor for sensing the process pressure in said outlet side and generating a signal proportional to said process pressure; control means connected to said signal generating means and responsive to said proportional signal for comparing said proportional signal with a pre-selected process pressure value, said control means generating a control signal representing the deviation of said proportional signal from said preselected process pressure value; and means responsive to the electrical current delivered to said motor and connected between said control means and said inlet valve, for passing said control signal without deviation to said inlet valve to adjust the inlet valve in response to said control signal so long as said motor current remains below a pre-selected value and modulating said control signal as a function of said motor current above said preselected value to adjust said inlet valve in response to said modulated control signal such that the maximum permissible current to said electric motor will not be exceeded.

2. The apparatus of claim 1, wherein said signal generating means comprises means for generating an air pressure signal proportional to said process pressure, and said control means is responsive to the air pressure signal; and further including a source of control air under pressure connected to said signal generating means. 1

3. The apparatus of claim 1, wherein said control means comprises means for generating an air pressure control signal and further including a source of control air under pressure connected to said control means.

4. The apparatus of claim 3, wherein said means re sponsive to the current delivered to said motor comprises a current-to-pressure converter whose output is an air pressure signal modulated as a function of said motor current above said pre-selected value, and further including a source of control air under pressure connected to said current-to-pressure" converter.

5. The apparatus of claim 4, further including means, through which said control signal must pass, connected intermediate said inlet valve and said current-topressure converter, for boosting the volume of said air pressure control signal without changing the pressure thereof, and a source of control air under pressure connected to said means for boosting the volume of said air pressure control signal.

6. The apparatus of claim 4, further including means for bypassing said control signal around said currentto-pressure converter to operate said inlet valve as a function of process pressure only in the event of a power failure.

7. The apparatus of claim 6, wherein said means for bypassing said control signal comprises: a normally closed, electrically operated valve through which said control signal must pass, connected intermediate said inlet valve and said current-to-pressure converter, said normally closed valve being electrically connected so as to be energized when electrical power is on; a bypass pipeline connected to the outlet of said control means and said inlet valve; and a normally open, electrically operated valve connected in said bypass pipeline intermediate said control means and said inlet valve, said normally open valve being electrically connected so as to be energized when electrical power is available.

8. Apparatus for preventing surge" in a centrifugal compressor driven by an electric motor, comprising, in combination: an adjustable blow-off valve connected to the outlet side of said compressor; means responsive to the electrical current delivered to said electric motor for generating a signal proportional to said motor current; and control means, connected to said means for generating a signal proportional to said motor current and said blow-off valve for generating a modulated control signal to open said blow-off valve as said proportional signal falls below a pre-selected value and close said blow-off valve as said proportional signal rises above said pre-selected value.

9. The apparatus of claim 8, wherein said means responsive to the electrical current delivered to said motor comprises a current-to-pressure converter for generating an air pressure signal proportional to said current, and said control means is responsive to the air pressure signal; and further including a source of control air under pressure connected to said current-topressure converter.

E0. The apparatus of claim 5, wherein said control means comprises means for generating an air pressure signal modulated as a function of said proportional sig nal and said pre-sel'ected minimum value of said proportional signal; and further including a source of control air under pressure connected to said control means.

11. The apparatus of claim 10, further including means intermediate said control means and said blowoff valve, for boosting the volume of said modulated air pressure signal without changing the pressure thereof, and a source of control air under pressure connected to said means for boosting the volume of said modulated air pressure signal.

12. The apparatus of claim 9, further including a normally closed, electrically operated valve through which said proportional air pressure signal must pass, connected intermediate said means responsive to the electrical current delivered to said motor and said control means for holding a constant air pressure signal into said control means during electrical power failures, said normally closed valve being electrically connected so as to be energized when electrical power is on.

13. An integrated, non-oscillatory, automatically temperature compensated flow control system for a centrifugal compressor, driven by an electric motor, for both limiting the current to said electric motor to a preselected maximum current and preventing the occurrence of surge in said compressor, comprising, in combination: an adjustable, flow-limiting inlet valve connected to the inlet side of said centrifugal compressor; signal generating means connected to the outlet side of said centrifugal compressor for sensing the process pressure in said outlet side and generating a signal proportional to said process pressure; first control means connected to said signal generating means and responsive to said proportional signal for comparing said proportional signal with a preselected process pressure value, said first control means generating a control signal representing the deviation of said proportional signal from said pre-selected process pressure value; first means responsive to the electrical current delivered to said motor and connected between said first control means and said inlet valve, for passing said control signal without deviation to said inlet valve to adjust said inlet valve in response to said control signal so long as said motor current remains below a preselected value and modulating said control signal as a function of said motor current above said pre-selected value to adjust said inlet valve in response to said modulated control signal such that the maximum permissible current to said electric motor will not be exceeded; an adjustable blow-off valve connected to the outlet side of said compressor; second means responsive to the electrical current delivered to said electric motor for generating a signal proportional to said motor current; and second control means, connected to said second means for generating a signal proportional to said motor current and said blow-off valve, for generating a modulated control signal to open said blow-off valve as said proportional signal falls below said pre-selected value and close said blow-off valve as said proportional signal rises above said pre-selected value.

14. The apparatus of claim 13, further including means for preventing the shutdown of said compressor during short-term power failures.

15. The apparatus of claim 14, wherein said means for preventing the shutdown of said centrifugal compressor comprises, in combination: a main electrical supply; a safety interlock circuit electrically connected to said main electrical supply for stopping said motor and for monitoring the status of critical components of said compressor and to prevent starting or continued operation of said motor in the event of an abnormal status of any one of said critical components; a motor starter circuit electrically connected to said main electrical supply and said drive motor for transferring electrical current to said motor; an enabling circuit electrically connected to and energized through said safety interlock circuit, said enabling circuit being electrically connected to said motor starter circuit for controlling said motor starter circuit to allow current to flow to said drive motor, said safety interlock circuit and said enabling circuit, upon failure of said main electrical supply, becoming de-energized to thereby open the connection of said main supply to said drive motor to prevent reconnection of said main electrical supply to said motor upon resumption of power to said main electrical supply; and circuit means connected to said main electrical supply and interconnecting said supply with said safety interlock circuit and said enabling circuit for automatically re-energizing said safety interlock circuit and said enabling circuit when the duration of the failure of said main electrical supply is of a duration less than a pre-determined period to thereby reestablish connection of said main electrical supply to said drive motor.

16. A method for limiting the flow through a centrifugal compressor, driven by an electric motor, to a preselected maximum value by controlling the degree of opening of an inlet valve on the inlet side of said centrifugal compressor, comprising the steps of:

a. sensing a process pressure in the outlet side of said centrifugal compressor;

b. generating a signal proportional to said process pressure;

c. comparing said proportional signal with a preselected process pressure;

d. generating a control signal proportional to the deviation of said proportional signal from said preselected process pressure;

e. sensing the current delivered to said electric motor;

f. comparing said control signal with a preselected maximum value of said motor current.

g. passing said control signal to said inlet valve without modification so long as said motor current does not exceed said pre-selected maximum value; and

h. modulating said control signal as a function of said motor current above said pre-selected maximum value, whereby the degree of opening of said inlet valve is directly related to said control signal.

17. A method for preventing surge" from occurring in a centrifugal compressor, driven by an electric motor, by opening a blow-off valve located in the outlet piping from said compressor comprising the steps of:

a. sensing the current delivered to said motor;

b. generating a signal proportional to said motor current;

c. comparing said proportional signal with a preselected minimum value of said proportional signal;

d. generating a modulated control signal as said proportional signal falls below said pre-selected minimum value; and

e. opening said blow-off valve as a function of said modulated control signal. 

1. Apparatus for limiting the current to an electric motor driving a centrifugal compressor to a pre-selected maximum current, comprising, in combination: An adjustable, flow-limiting inlet valve connected to the inlet side of said centrifugal compressor; signal generating means connected to the outlet side of said centrifugal compressor for sensing the process pressure in said outlet side and generating a signal proportional to said process pressure; control means connected to said signal generating means and responsive to said proportional signal for comparing said proportional signal with a pre-selected process pressure value, said control means generating a control signal representing the deviation of said proportional signal from said pre-selected process pressure value; and means responsive to the electrical current delivered to said motor and connected between said control means and said inlet valve, for passing said control signal without deviation to said inlet valve to adjust the inlet valve in response to said control signal so long as said motor current remains below a pre-selected value and modulating said control signal as a function of said motor current above said pre-selected value to adjust said inlet valve in response to said modulated control signal such that the maximum permissible current to said electric motor will not be exceeded.
 2. The apparatus of claim 1, wherein said signal generating means comprises means for generating an air pressure signal proportional to said process pressure, and said control means is responsive to the air pressure signal; and further including a source of control air under pressure connected to said signal generating means.
 3. The apparatus of claim 1, wherein said control means comprises means for generating an air pressure control signal and further including a source of control air under pressure connected to said control means.
 4. The apparatus of claim 3, wherein said means responsive to the current delivered to said motor comprises a ''''current-to-pressure'''' converter whose output is an air pressure signal modulated as a function of said motor current above said pre-selected value, and further including a source of control air under pressure connected to said ''''current-to-pressure'''' converter.
 5. The apparatus of claim 4, further including means, through which said control signal must pass, connected intermediate said inlet valve and said ''''current-to-pressure'''' converter, for boosting the volume of said air pressure control signal without changing the pressure thereof, and a source of control air under pressure connected to said means for boosting the volume of said air pressure control signal.
 6. The apparatus of cLaim 4, further including means for bypassing said control signal around said ''''current-to-pressure'''' converter to operate said inlet valve as a function of process pressure only in the event of a power failure.
 7. The apparatus of claim 6, wherein said means for bypassing said control signal comprises: a normally closed, electrically operated valve through which said control signal must pass, connected intermediate said inlet valve and said ''''current-to-pressure'''' converter, said normally closed valve being electrically connected so as to be energized when electrical power is on; a bypass pipeline connected to the outlet of said control means and said inlet valve; and a normally open, electrically operated valve connected in said bypass pipeline intermediate said control means and said inlet valve, said normally open valve being electrically connected so as to be energized when electrical power is available.
 8. Apparatus for preventing ''''surge'''' in a centrifugal compressor driven by an electric motor, comprising, in combination: an adjustable blow-off valve connected to the outlet side of said compressor; means responsive to the electrical current delivered to said electric motor for generating a signal proportional to said motor current; and control means, connected to said means for generating a signal proportional to said motor current and said blow-off valve for generating a modulated control signal to open said blow-off valve as said proportional signal falls below a pre-selected value and close said blow-off valve as said proportional signal rises above said pre-selected value.
 9. The apparatus of claim 8, wherein said means responsive to the electrical current delivered to said motor comprises a ''''current-to-pressure'''' converter for generating an air pressure signal proportional to said current, and said control means is responsive to the air pressure signal; and further including a source of control air under pressure connected to said ''''current-to-pressure'''' converter.
 10. The apparatus of claim 8, wherein said control means comprises means for generating an air pressure signal modulated as a function of said proportional signal and said pre-selected minimum value of said proportional signal; and further including a source of control air under pressure connected to said control means.
 11. The apparatus of claim 10, further including means intermediate said control means and said blow-off valve, for boosting the volume of said modulated air pressure signal without changing the pressure thereof, and a source of control air under pressure connected to said means for boosting the volume of said modulated air pressure signal.
 12. The apparatus of claim 9, further including a normally closed, electrically operated valve through which said proportional air pressure signal must pass, connected intermediate said means responsive to the electrical current delivered to said motor and said control means for holding a constant air pressure signal into said control means during electrical power failures, said normally closed valve being electrically connected so as to be energized when electrical power is on.
 13. An integrated, non-oscillatory, automatically temperature compensated flow control system for a centrifugal compressor, driven by an electric motor, for both limiting the current to said electric motor to a pre-selected maximum current and preventing the occurrence of ''''surge'''' in said compressor, comprising, in combination: an adjustable, flow-limiting inlet valve connected to the inlet side of said centrifugal compressor; signal generating means connected to the outlet side of said centrifugal compressor for sensing the process pressure in said outlet side and generating a signal proportional to said process pressure; first control means connected to said signal generating means and responsive to said proportional signal for comparing said proportional signal with a preselected process pressure value, said first control means generating a control signal representing the deviation of said proportional signal from said pre-selected process pressure value; first means responsive to the electrical current delivered to said motor and connected between said first control means and said inlet valve, for passing said control signal without deviation to said inlet valve to adjust said inlet valve in response to said control signal so long as said motor current remains below a pre-selected value and modulating said control signal as a function of said motor current above said pre-selected value to adjust said inlet valve in response to said modulated control signal such that the maximum permissible current to said electric motor will not be exceeded; an adjustable blow-off valve connected to the outlet side of said compressor; second means responsive to the electrical current delivered to said electric motor for generating a signal proportional to said motor current; and second control means, connected to said second means for generating a signal proportional to said motor current and said blow-off valve, for generating a modulated control signal to open said blow-off valve as said proportional signal falls below said pre-selected value and close said blow-off valve as said proportional signal rises above said pre-selected value.
 14. The apparatus of claim 13, further including means for preventing the shutdown of said compressor during short-term power failures.
 15. The apparatus of claim 14, wherein said means for preventing the shutdown of said centrifugal compressor comprises, in combination: a main electrical supply; a safety interlock circuit electrically connected to said main electrical supply for stopping said motor and for monitoring the status of critical components of said compressor and to prevent starting or continued operation of said motor in the event of an abnormal status of any one of said critical components; a motor starter circuit electrically connected to said main electrical supply and said drive motor for transferring electrical current to said motor; an enabling circuit electrically connected to and energized through said safety interlock circuit, said enabling circuit being electrically connected to said motor starter circuit for controlling said motor starter circuit to allow current to flow to said drive motor, said safety interlock circuit and said enabling circuit, upon failure of said main electrical supply, becoming de-energized to thereby open the connection of said main supply to said drive motor to prevent reconnection of said main electrical supply to said motor upon resumption of power to said main electrical supply; and circuit means connected to said main electrical supply and interconnecting said supply with said safety interlock circuit and said enabling circuit for automatically re-energizing said safety interlock circuit and said enabling circuit when the duration of the failure of said main electrical supply is of a duration less than a pre-determined period to thereby re-establish connection of said main electrical supply to said drive motor.
 16. A method for limiting the flow through a centrifugal compressor, driven by an electric motor, to a pre-selected maximum value by controlling the degree of opening of an inlet valve on the inlet side of said centrifugal compressor, comprising the steps of: a. sensing a process pressure in the outlet side of said centrifugal compressor; b. generating a signal proportional to said process pressure; c. comparing said proportional signal with a preselected process pressure; d. generating a control signal proportional to the deviation of said proportional signal from said preselected process pressure; e. sensing the current delivered to said electric motor; f. comparing said control signal with a preselected maximum value of said motor current. g. passing said control signal to said inlet valve without modification so long as said motor current does not exceed said pRe-selected maximum value; and h. modulating said control signal as a function of said motor current above said pre-selected maximum value, whereby the degree of opening of said inlet valve is directly related to said control signal.
 17. A method for preventing ''''surge'''' from occurring in a centrifugal compressor, driven by an electric motor, by opening a blow-off valve located in the outlet piping from said compressor comprising the steps of: a. sensing the current delivered to said motor; b. generating a signal proportional to said motor current; c. comparing said proportional signal with a pre-selected minimum value of said proportional signal; d. generating a modulated control signal as said proportional signal falls below said pre-selected minimum value; and e. opening said blow-off valve as a function of said modulated control signal. 